1. Field of Invention
The present invention relates to a time correction device for setting the time based on a signal from a positioning satellite such as a GPS satellite, to a timepiece having the time correction device, and to a method of correcting the time.
2. Description of the Related Art
The Global Position System (GPS), which is a system enabling a receiving device to determine its own position, uses satellites (GPS satellites) that each orbit the Earth on a specific path and have an on-board atomic clock. Each GPS satellite therefore keeps time (GPS time) with extremely high precision.
In order for a receiver (GPS receiver) that receives a signal from a GPS satellite to get the time information transmitted from the GPS satellite, and more particularly to get time information that is precise to at least 1 ms, the GPS receiver must usually receive the TOW (Time of Week) signal transmitted by the GPS receiver. The TOW signal denotes the number of seconds form the beginning of the week, and is reset every week.
The TOW signal is carried in each subframe of the GPS signal. The subframe signal must therefore be received in order to get the TOW signal, and it takes 6 seconds to receive one subframe.
Japanese Patent No. 3512068 (JP2001-59864) (see FIG. 5) teaches a method of synchronizing the receiver with the GPS satellite time without receiving the TOW signal.
The method taught in Japanese Patent No. 3512068 is a method of synchronizing to the time information required for positioning, and the TOW data is not needed for the time synchronization that is required for positioning.
However, if the GPS time transmitted from the GPS satellites is to be used to set the time of a timepiece, the TOW data must be received and receiving the TOW signal requires the six seconds needed to receive a subframe. Furthermore, the TOW data should be received twice in order to ensure that the TOW data was correctly received, and receiving the TOW data twice requires 12 seconds.
The receiver power must remain on during TOW signal reception, and this increases power consumption. Incorporating a receiver with such high power consumption in a timepiece or similar device with extremely low power requirements is thus impractical, and such a receiver therefore cannot be used to set the time with high precision in a timepiece such as a wristwatch or other similarly small electronic device.